1. Technical Field
This invention relates to the field of sound transducers, and more particularly to a method and apparatus for sensing sound waves in a fluid medium.
2. Description of Related Art
Hydrophones are transducers which generate an output in response to sound waves which are propagating through a fluid medium. Because they are able to detect sound waves in this manner, they often are used in a wide variety of applications. When used in seismic exploration, an array of hydrophones are generally towed by a ship. In addition to the array of hydrophones, the ship generally tows a sound source which emits periodic pulses of acoustic energy. These pulses emitted by the sound source travel through the water and are reflected back to the array of hydrophones from the interfaces between the layers of sediment and rock. After the reflected pulses are received by the hydrophones, the outputs from the hydrophones are amplified and processed so as to produce a cross-sectional view of the sediments on the ocean floor.
Hydrophones may also be used in underwater communication where direct wire links are disadvantageous and radio wave propagation is generally difficult. In such underwater communication, a carrier wave of a fixed frequency is amplitude modulated by a voice signal. The modulated signal is delivered to a projector which is used to emit sound waves in water in response to the modulated signal. A hydrophone is then used to receive the sound waves generated by the projector. The output of the hydrophone is then demodulated so that the original voice signal may be reproduced.
Finally, hydrophones may be used to detect the presence of submarines. When used for this purpose, a linear array of hydrophones are generally towed behind a ship. Each of the hydrophones in the array attempt to sense sound which may indicate the presence of a submarine.
While operation of hydrophones will vary depending on the specific structure used as well as the nature of the application, perhaps the most general approach to hydrophone operation is as follows. When sound waves encounter a hydrophone, they are delivered to an electrorestrictive material such as barium titanate and lead metaniobate. These electrorestrictive materials have the property that they generate an electric voltage when they are deformed by the force induced by the sound waves acting on the hydrophone. By measuring the voltage generated by the electrorestrictive material, a signal responsive to the sound waves may be generated.
There are several disadvantages which are often associated with the type of hydrophone described above. Since they often involve a relatively large number of metal components, this type of hydrophone is often susceptible to failure due to excessive corrosion. In addition, such hydrophones are often limited in terms of the static and dynamic pressure ranges in which they can operate, as well as in terms of frequency response. Further, because such hydrophones have a relatively large number of components, they are often relatively expensive. Finally, because such hydrophones often generate an electrical signal by electromechanical techniques, they often generate a pulse of sound when they are activated which may jeopardize security in an otherwise secure environment.